OF GUN-METAL.

144

elevated. The visual ray should pass over the ornamentsand intersect the axis of the piece produced not far fromthe muzzle (94). From the construction of these shortguns, there is a very great difference between the dia-meters of the base-ring and muzzle, which being consideredby some artillerists as a defect, a projection was made uponthe base-ring of some guns cast in the last century, andpierced with holes; which gave the gunner a free and unin-terrupted view along the gun, and enabled him to lay it withprecision to very distant objects : others have imagined amoveable instrument pierced with holes, the base resting ver-tically upon the base ring, by means of which the gun couldbe laid at different elevations as the visual ray passed throughthe holes along the gun towards the object; other holes weremade horizontally to correct the direction in cafe the precedingone was wrong: but as artillerists may supply the want of theseinstruments in a very simple and easy manner, it is unnecessaryto dwell any longer on the subject.

99. The thicknesses of metal in mortars are the same aswere used by our ancestors for a long series of years. Thetheorem 9962 nr' L = 2rra + »! 1 x q will shew that the thick-ness at the breech in mortars with spherical chambers is suf-ficient to resist the strongest pressures of the fluid, when thechamber is filled with fine w ar powder confined by a tompionof wood, and the shell surrounded with earth strongly com-pressed. The theofem (Philos. Instit.) for the bases of cy-linders will serve for mortars with cylindric chambers, but asthe modern powder is much stronger than that used in thelast century, the thicknesses may be increased -J, and then themortars will be sufficiently strong for every purpose.

100. In mortars fired with the largest charges, the elasticfluid generated in the chamber is considerably dilated onpassing into the chafe; the scale of pressures therefore willvery suddenly approach the axis of the mortar ; whence nodamage can ensueif the thicknesses have been properly pro-portioned. Mortars cast on these proportions, of metalcontaining 4 of tin, resist the pressure of the fluid, andevery other force that tends to destroy them, as the impulsionof the fluid, and the percussion of the shell: but if thechafe be not united to the breech by a considerable thicknessof metal, it will be broken off, or at least incurvated. Ifthe metal contain of tin, the impression of the shells willbe less ; but the thickness must be increased.

101. Mortars